Field of the Invention
The invention relates to a unit for detecting the position of a moving part, said unit having reduced offset voltage and comprising a position sensor of the inductive type and apparatuses for processing the signal of this sensor. The voltage measurement error of a sensor, referred to as the offset voltage, corresponds to the difference between the measured value of the position of the moving part and the actual position of this part.
Thus, in the primary application, but not exclusively, the invention relates to the improvement of the position measurement given by rotary sensors. These rotary sensors have numerous applications in the field of industrial automation, in particular each time it is necessary to monitor, with sufficient accuracy, the position of a moving mechanical element. For example, the monitoring of the angular position of a motor turning at up to 15000 revolutions per minute is systematic.
Description of the Related Art
Position sensors of the inductive type give the position of mechanical parts thanks to the analysis of electrical currents or voltages that vary depending on the displacement of a moving part element. This moving element may be either the part itself or a moving element referred to as a target, which is fixed on the part, wherein it is sought to know the position of said moving element. The electrical voltages to be analyzed are induced, in the case of induction flat panel detectors, by a high-frequency alternating current passing through a fixed circuit, referred to as a primary circuit, generally provided in loop form on a flat surface. This high-frequency alternating current induces a magnetic field at the same frequency as the current passing through it, in at least one secondary circuit.
The secondary circuits are also fixed. Each secondary circuit is placed on a flat surface close to that of the primary circuit, thus forming at least two loops for which the couplings between the primary circuit and the loops of the secondary circuits create a magnetic flux in each of the loops of the secondary circuits. The moving target cuts the passage of the magnetic flux, and therefore the voltage induced by the magnetic field in the loops varies depending on the position of the target in front of these loops.
The induction sensor may have a planar construction, but also an axial construction, with the primary circuit and the secondary circuits in the form of solenoids, and the target, which is cylindrical, which moves back and forth between the solenoid of the primary circuit and the solenoids of the secondary circuits. The result in terms of analysis of the voltages at the terminals of the secondary circuits is the same.
Voltage measurements are taken at the terminals of the secondary windings, and the measured voltages describe curves similar to sine curves, between a negative minimum value and a positive maximum value. In the conventional case, in which two secondary circuits are positioned, the curve of one secondary winding is that of a sine, and that of the other secondary winding is that of a cosine.
The result of the voltages received at the terminals of the secondary circuits is first processed so as to be shaped: firstly, a demodulation removes the high-frequency component caused by the current of the primary circuit, then a calculation on the basis of the signals referred to as sine and cosine signals, from the secondary circuits, provides the angular position of the target with the aid of an arctangent function.
The processing of these signals is described for example in patent documents FR 2 542 468, EP 0 182 085 or EP 0 468 642. It utilizes primarily the following apparatuses between the output terminals of the secondary windings and the output terminals of the sensor:                an amplifier placed at the terminals of each secondary circuit, intended to amplify the signal received from one or the other secondary circuit; preferably one amplifier per secondary circuit processes the two circuits simultaneously, which makes it possible to avoid any delay of the processing of the signals;        a signal shaping circuit with demodulation and low-pass filter; the demodulation is carried out by multiplexers connected via interrupters (also referred to as switches). At the output of this shaping circuit, a signal in sine form is thus obtained corresponding to one secondary circuit, and a signal in cosine form is thus obtained for the other secondary circuit.        
These sinusoidal signals, which represent the displacement of the target, are deformed by parasitic pulses (see references 10, 11 and 13 in FIG. 2). These deformations manifest themselves by parasitic signals in the sinusoidal curves as illustrated by FIGS. 1 and 2 hereinafter, which leads to errors in the calculation of the angular position of the target. These parasitic signals 10, 11 and 13 are inherent to the design of the sensors according to the prior art, and are caused primarily by the two following apparatuses: the amplifier, which amplifies both the measured value and the errors, and the switches serving as demodulator. In fact, one of the outputs of these switches is not connected to any element, which injects parasitic pulses into the servo loop of the amplifier when the multiplexer is connected in this loop. These parasitic signals create measurement errors.